Forward osmosis has been used for desalination. In general, a forward osmosis desalination process involves a container having two chambers separated by a semi-permeable membrane. One chamber contains seawater. The other chamber contains a concentrated solution that generates a concentration gradient between the seawater and the concentrated solution. This gradient draws water from the seawater across the membrane, which selectively permits water to pass, but not salts, into the concentrated solution. Gradually, the water entering the concentrated solution dilutes the solution. The solutes are then removed from the dilute solution to generate potable water.
One drawback to forward osmosis systems is the ion exchange phenomena, which disturbs the ion balance of the system. For example, in a system using a NH3—CO2 draw solution and a NaCl feed solution, Na+ and NH4+ ions will exchange across the membrane, which can result in a higher salinity product water and increased difficulty in recovering draw solutes. Some desalination units currently use pre and post-treatment ion exchange or similar processes; however, that use is typically done to further condition a product solvent and not in an attempt to overcome these drawbacks, in particular with respect to recovering draw solutes.